This invention relates to electrostatographic toner image reproduction machines, and more particularly to such a machine including a method and apparatus for stabilizing productivity in the face of declining toner concentration, thereby deterring dead cycling and thus assuring operator satisfaction.
In electrostatographic toner image reproduction machines such as copiers and printers, toner reproductions are made using toner particles, contained in developer material at a desired concentration level. As toner particles are depleted from the developer material, additional toner particles must be added thereto in order to maintain the toner concentration at the desired level. Typically, the toner concentration of a machine is monitored by suitable means, and is maintained by adding fresh toner particles to the development housing of the machine.
For monitoring and maintaining the toner concentration of such a machine, many types of systems including high cost toner concentration sensors, have been proposed. For example, U.S. Pat. No. 4,619,522 to Imai teaches the use of a reference pattern, with a predetermined reflectance, that is developed. Subsequently, the density of the developed pattern is detected by a sensor, and used to regulate the replenishment of toner to the developer housing.
Furthermore, U.S. Pat. No. 4,434,221 to Oka discloses a method of utilizing a reference latent image to measure the current flow between the developing sleeve and the photoreceptor drum during development of the reference image. Subsequently, the amount of toner needed for replenishment is controlled, based on the current value measured. Oka further characterizes this method as inferior, because, the variation in current value due to toner concentration is exceeded by the variation due to the amount of toner adhering to the reference image.
In addition, U.S. Pat. No. 4,492,179 to Folkins et al., teaches the sensing of the charge of the toner particles being transferred to the latent image, and means for controlling the addition of toner to the developer housing as a function of that measurement. Folkins et al. also discloses the limitations of the marking particle dispense control system, relating to toner dispensing assumptions, in which the rate of dispense must remain constant over the life of the system. More specifically, any variation in the toner mass dispensed for a given electrical input will manifest itself proportionally as a shift in the relationship between the toner dispense rate and the bias current required for the developed toner charge.
Unfortunately however, toner depletion or consumption can and often outstrips toner replenishment particularly when running long jobs with relatively high toner area coverage. Typically, the response of conventional machines is to dead cycle, or to skip a bunch of pitches when a certain trigger is reached. This approach has been found to cause operator dissatisfaction. In other words, in existing xerographic print engines, when a control point falls too far from target and print quality is expected to suffer, the print engine goes into a dead cycle mode turning off customer prints while the system recovers using normal or accelerated process controls.
There is therefore a need for an electrostatographic toner image reproduction machines, and more particularly to such a toner image reproduction machine having a distributed pitch skipping method and apparatus for preventing dead cycling and thus assuring operator satisfaction.